Alteration of antibiotic-resistant phenotypes and minimal inhibitory concentration of Escherichia coli in pig farming: Comparison between closed and open farming systems

Highlights: • Open (OSF) and closed (CSF) pig farms were studied for 7-antibiotics resistance and 21-antibiotics prevalence. • OSF showed higher resistant E. coli and antibiotics contamination in all samples. • Resistant E. coli phenotype was not correlated with residual antibiotics. • CSF had not used CLT, but CLT-resistant E. coli developed in wastewater treatment system with high MIC. The prevalence of antibiotic residues and antibiotic-resistant Escherichia coli (ARE) in closed (CSF) and open (OSF) pig farming systems was analysed. Results showed that a high level of E. coli populations, antibiotic contamination in supplied water, and additional antibiotics used, such as neomycin (NEO) or colistin (CLT), were observed in OSF. Similar ARE resistance to five antibiotics, including NEO, was observed in the water source, dung, and wastewater/sludge but were different from those of CLT. An increased occurrence of ARE was found in the sludge of anaerobic digestion (AD) and the waste stabilisation pond (WSP), but they were not well correlated with their residual antibiotic concentrations. CLT administration yielded higher ARE prevalence in pig excreta and wastewater in OSF, but its absence in CSF also resulted in ARE occurrence with increased minimal inhibitory concentration (MIC) levels in the anaerobic digester/waste stabilisation ponds. This study revealed that ARE prevalence and MIC levels of CLT could be developed during the wastewater treatment process in the pig farm, although none of the AREs were found in the original excreta/wastewater.